Readily available from crop residues and forests, cellulosic biomass is a largely untapped resource for fuels and chemicals. [Perlack et al., “Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply,” U.S. Department of Energy and U.S. Department of Agriculture, Oak Ridge National Laboratory: Oak Ridge, Tenn. DOE/GO-102995-2135 (2005).] Efficient transformations of biomass polymers into small molecules will enable the development of a renewable chemical and fuel industry. [Werpy et al., “Top Value-Added Chemicals from Biomass, Volume I: Results of Screening for Potential Candidates from Sugars and Synthesis Gas,” U.S. Department of Energy, Office of Scientific and Technical Information: Oak Ridge, Tenn. DOE/GO-102004-1992 (2004).] Useful small molecules that can be obtained from biomass include furans, such as 5-hydroxyfurfural and furfural.
A hexose dehydration product, 5-hydroxymethylfurfural (HMF), is a six-carbon molecule analogous to commodity chemicals like adipic acid and terephthalic acid. As shown in FIG. 1, HMF can be converted by straightforward methods into a variety of useful acids, aldehydes, alcohols, and amines, as well as the promising fuel, 2,5-dimethylfuran (DMF). [Lichtenthaler, Acc. Chem. Res. 35, 728-737 (2002); Lewkowski, ARKIVOC, 17-54 (2001).] The energy content of DMF (31.5 MJ/L) is similar to that of gasoline (35 MJ/L) and 40% greater than that of ethanol (23 MJ/L). [Nisbet, H. B. J. Inst. Petrol. 1946, 32, 162-166; Roman-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Nature 2007, 447, 982-986.] Moreover, DMF (bp 92-94° C.) is less volatile than ethanol (bp 78° C.) and is immiscible with water making DMF an attractive alternative liquid fuel for transportation.
HMF synthesis has been demonstrated in water [Lewkowski, ARKIVOC, 17-54 (2001).], traditional organic solvents [Halliday et al., Org. Lett. 5, 2003-2005 (2003)], multiphase systems [Roman-Leshkov et al., Science 312, 1933-1937 (2006); Chheda et al., Green Chemistry 9, 342-350 (2007).], and ionic liquids [Zhao et al., Science 316, 1597-1600 (2007); Moreau et al., J. Mol. Catal. A: Chem. 253, 165-169 (2006); Moreau et al., Catal. Commun. 4, 517-520 (2003).]. In a typical process, acid catalysts are used to dehydrate fructose to make HMF as shown in FIG. 2. This figure also schematically illustrates the potential for formation of undesired side-products and that HMF itself can be further degraded.
Nevertheless, several underlying factors have blocked the economical production of HMF and HMF-derivatives for over a century: the requirement for expensive fructose feedstock, the low selectivity of fructose dehydration in water, and the difficulty of purifying HMF from the few suitable organic solvents in which the process can be conducted.
High HMF yields (up to 85% molar yield) from fructose are reported for mixed aqueous-organic biphasic systems containing polar organic solvents [Roman-Leshkov et al., Science 312, 1933-1937 (2006); Chheda et al., Green Chemistry 9, 342-350 (2007); Dumesic et al., published application US 2008/0033188 (2008)]. This work describes a biphasic reaction process for making furan derivatives involving dehydrating a feedstock solution containing a carbohydrate in the presence of an acid catalyst. The reaction is conducted in a reaction vessel containing a biphasic reaction medium containing an aqueous reaction solution and a substantially immiscible organic extraction solution. The biphasic medium contains a modifier in the aqueous phase, in the organic phase or both to improve selectivity of the process to yield furan derivative compounds. The aqueous phase modifier can be a metal salt, such as NaCl or a dipolar aprotic species including dimethylsulfoxide, dimethylformamide, N-methylpyrrolidinone, acetonitrile, butryolactone, dioxane or pyrrolidinone. Catalysts reported include HCl, H2SO4, and H3PO4. Although high molar yields are reported for conversion of fructose to HMF, the highest yields from glucose do not exceed 30% and conversion of cellulose to HMF is not reported. The patent application also reports conversion of HMF to DMF using CuCrO4 or Cu:Ru on carbon.
Other recent reports describe conversion of fructose, glucose, and cellulose into HMF in ionic liquids [Zhao et al., Science 316, 1597-1600 (2007); Zhao et al., published application US 2008/0033187 (2008)]. Zhao et al. report a method for conversion of a carbohydrate in an ionic liquid producing a furan at a substantial yield. The method involves mixing carbohydrate up to the limit of solubility with the ionic liquid, and heating the carbohydrate in the presence of a catalyst at a reaction temperature and for a reaction time sufficient for conversion to furan at a substantial yield. Exemplary ionic liquids are [EMIM]Cl (1-ethyl-3-methyl-imidazolium chloride) and [BMIM]Cl (1-butyl-3-methyl-imidazolium chloride). The reference reports conversion of fructose to HMF in ionic liquid in the presence of metal halides and acid catalysts and the conversion of glucose to HMF in ionic liquid in the presence of chromium chloride catalyst. Ionic liquids are expensive, and product is difficult to purify.
In the past, monosaccharides such as fructose and glucose have been the primary feedstocks for synthesis of HMF. Polysaccharides such as cellulose are another type of feedstock for HMF synthesis which are readily and inexpensively available. Despite this potential, HMF is not typically obtained from cellulose in high yield. Aqueous acid and high temperatures and pressures (250-400° C., 10 MPa) enable conversion of cellulose into HMF and levulinic acid with a 30% molar yield of HMF [Kono et al., published application Jap. 2005232116 (2005)]. In alkylimidazolium chloride ionic liquids, chromium chlorides are reported to catalyze the conversion of cellulose into HMF in 51% molar yield [Zhao et al., published application US 2008/0033187 (2008)]. The first method requires harsh conditions which increase process costs, and the second method uses expensive ionic liquid solvents.
Most efforts toward HMF production have used edible starting materials, primarily fructose and glucose. In fact, almost all renewable fuels and chemicals are usually based on food resources such as starch, sugars, and oils. These simple starting materials are relatively easy to convert into valuable products, while inedible lignocellulosic biomass is relatively recalcitrant and heterogeneous, making its conversion typically inefficient and uneconomical. [Zhang, Y.-H. P.; Ding, S.-Y.; Mielenz, J. R.; Cui, J.-B.; Elander, R. T.; Lasser, M.; Himmel, M. E.; McMillan, J. R.; Lynd, L. R. Biotechnol. Bioeng. 2007, 97, 214-223.]
Both xylan and xylose can be dehydrated into furfural, a biofuel precursor and industrial chemical. [Zeitsch, K. J. (2000) The Chemistry and Technology of Furfural and Its Many By-Products Elsevier: Amsterdam; Lee, J.-M.; Kim, Y.-C.; Hwang, I. T.; Park, N.-J.; Hwang, Y. K.; Chang, J.-S. (2008) Biofuels, Bioproducts, and Biorefining, 2, 438-454.] Furfural is perhaps the most common industrial chemical derived from lignocellulosic biomass with annual production of more than 200,000 t. [Kamm, B.; Gruber, P. R.; Kamm, M., Eds (2006) Biorefineries—Industrial Processes and Products, Wiley—VCH: Weinheim, Germany.] The conversion of pentoses into furfural has been reported. [Sproull, R. D.; Bienkowski, P. R.; Tsao, G. T. (1985) Biotechnology and Bioengineering Symposium, 15, 561-577; Moreau, C.; Durand, R.; Peyron, D.; Duhamet, J.; Rivalier, P. (1988) Industrial Crops and Products, 7, 95-99; Mansilla, H. D.; Baeza, J.; Urzua, S.; Maturana, G.; Villasenor, J.; Duran, N. (1998) Bioresource Technology 66, 189-193; Dias, A. S.; Lima, S.; Pillinger, M.; Valente, A. A. (2007) Catalysis Letters, 114, 151-160.] Most industrial processes achieve yields in the range of 50 molar %, which may be limited by homopolymerization and condensation with unreacted xylose. In typical processes reported, Brønsted acidic catalysts were used in aqueous solution at temperatures greater than 150° C. [Moreau, C.; Durand, R.; Peyron, D.; Duhamet, J.; Rivalier, P. (1998) Industrial Crops and Products, 7, 95-99; Dias, A. S.; Lima, S.; Pillinger, M.; Valente, A. A. (2007) Catalysis Letters, 114, 151-160.] FIG. 3 illustrates mechanisms proposed for the formation of furfural from xylan (A) and xylose (B). In this mechanism, the C-2 hydroxyl group is displaced to form a xylose-2,5-anhydride and subsequent dehydration steps produce furfural. [Antal, M. J.; Richards, G. N. (1991) Carbohydrate Research 217, 71-85; Nimlos, M. R.; Qian, X.; Davis, M.; Himmel, M. E.; Johnson, D. K. (2006) Journal of Physical Chemistry A 110, 11824-11838.]
There remains a need in the art for efficient and lower cost methods for conversion of biomass, including lignocellulosic biomass, to useful fuels and chemicals. This invention provides methods for conversion of biomass to furans, particularly to HMF and furfural, and to the potentially important fuel component, DMF (2,5-dimethylfuran) which employ polar aprotic solvents in the presence of certain salts.